Investigations undertaken during the past several years identified optimal subsystem designs for a stationary ring, small animal positron emission tomography (PET) scanner with depth-of-interaction capability. During this reporting period the first of two ATLAS (Advanced Technology Laboratory Animal Scanner) scanners based on these designs was mechanically and electronically completed. The second system, ATLAS II, was mechanically completed and all components including custom electronics boards and amplifier packages were fabricated including the 18 detector modules that surround the animal. The first and second systems differ from one another only in that the second system is based on Hamamatsu R7600 C-12 position-sensitive photomultiplier tubes (PSPMTs) whereas the first system is based on the R7600 C-8 PSPMTs. This difference will allow ATLAS II (primarily a physics research machine) to be eventually upgraded to higher spatial resolution and in the meantime provide the Imaging Physics Laboratory with a research test bed for continuing experiments in high resolution PET. ATLAS I is intended ultimately for routine use in the intramural research program to study organ function in small animals such as rats and genetically altered mice. Preliminary measurement made on ATLAS I indicate that the performance predictions made by computer simulation of the ATLAS design are accurate and that ATLAS meets virtually all of the design goals set for this system. Among these are the highest absolute central point source sensitivity of any system of comparable axial field-of-view and a resolution uniformity superior to any system of comparable ring diameter. During this reporting period work commenced on a comprehensive software package and user interface for ATLAS that includes a number of data acquisition protocols, a variety of local and remote image reconstruction options and an extended image analysis and visualization package that includes multi-modality image registration software. This latter capability will be exploited when the small animal CT scanner ordered this year is delivered and integrated into the study environment during the next reporting period. CT images obtained on each animal undergoing a PET study will be used to correct the PET image data for attenuation and to help identify structures labeled with the PET radiopharmaceutical.